What is Kafka?

Apache Kafka is a distributed streaming platform used for building real-time data pipelines and streaming apps. It is horizontally scalable, fault-tolerant, wicked fast, and runs in production in thousands of companies. Kafka requires a connection to a Zookeeper service.

https://kafka.apache.org/

TL;DR;

Docker Compose

version: '2'
services:
  zookeeper:
    image: 'bitnami/zookeeper:latest'
    ports:
      - '2181:2181'
    environment:
      - ALLOW_ANONYMOUS_LOGIN=yes
  kafka:
    image: 'bitnami/kafka:latest'
    ports:
      - '9092:9092'
    environment:
      - KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181
      - ALLOW_PLAINTEXT_LISTENER=yes

Why use Bitnami Images?

Bitnami closely tracks upstream source changes and promptly publishes new versions of this image using our automated systems.
With Bitnami images the latest bug fixes and features are available as soon as possible.
Bitnami containers, virtual machines and cloud images use the same components and configuration approach - making it easy to switch between formats based on your project needs.
All our images are based on minideb a minimalist Debian based container image which gives you a small base container image and the familiarity of a leading linux distribution.
Bitnami container images are released daily with the latest distribution packages available.

This CVE scan report contains a security report with all open CVEs. To get the list of actionable security issues, find the "latest" tag, click the vulnerability report link under the corresponding "Security scan" field and then select the "Only show fixable" filter on the next page.

How to deploy Apache Kafka in Kubernetes?

Deploying Bitnami applications as Helm Charts is the easiest way to get started with our applications on Kubernetes. Read more about the installation in the Bitnami Apache Kafka Chart GitHub repository.

Bitnami containers can be used with Kubeapps for deployment and management of Helm Charts in clusters.

Why use a non-root container?

Non-root container images add an extra layer of security and are generally recommended for production environments. However, because they run as a non-root user, privileged tasks are typically off-limits. Learn more about non-root containers in our docs.

Supported tags and respective `Dockerfile` links

NOTE: Debian 8 images have been deprecated in favor of Debian 9 images. Bitnami will not longer publish new Docker images based on Debian 8.

Learn more about the Bitnami tagging policy and the difference between rolling tags and immutable tags in our documentation page.

Subscribe to project updates by watching the bitnami/kafka GitHub repo.

Get this image

The recommended way to get the Bitnami Kafka Docker Image is to pull the prebuilt image from the Docker Hub Registry.

docker pull bitnami/kafka:latest

To use a specific version, you can pull a versioned tag. You can view the list of available versions in the Docker Hub Registry.

docker pull bitnami/kafka:[TAG]

If you wish, you can also build the image yourself.

docker build -t bitnami/kafka:latest https://github.com/bitnami/bitnami-docker-kafka.git

Persisting your data

If you remove the container all your data and configurations will be lost, and the next time you run the image the database will be reinitialized. To avoid this loss of data, you should mount a volume that will persist even after the container is removed.

Note! If you have already started using your database, follow the steps on backing up and restoring to pull the data from your running container down to your host.

The image exposes a volume at /bitnami/kafka for the Kafka data. For persistence you can mount a directory at this location from your host. If the mounted directory is empty, it will be initialized on the first run.

Using Docker Compose:

version: '2'

services:
  zookeeper:
    image: 'bitnami/zookeeper:latest'
    ports:
      - '2181:2181'
  kafka:
    image: 'bitnami/kafka:latest'
    ports:
      - '9092:9092'
    volumes:
      - /path/to/kafka-persistence:/bitnami/kafka

Connecting to other containers

Using Docker container networking, a Kafka server running inside a container can easily be accessed by your application containers.

Containers attached to the same network can communicate with each other using the container name as the hostname.

Using the Command Line

In this example, we will create a Kafka client instance that will connect to the server instance that is running on the same docker network as the client.

Step 1: Create a network

$ docker network create app-tier --driver bridge

Step 2: Launch the Zookeeper server instance

Use the --network app-tier argument to the docker run command to attach the Zookeeper container to the app-tier network.

$ docker run -d --name zookeeper-server \
    --network app-tier \
    bitnami/zookeeper:latest

Step 2: Launch the Kafka server instance

Use the --network app-tier argument to the docker run command to attach the Kafka container to the app-tier network.

$ docker run -d --name kafka-server \
    --network app-tier \
    -e KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181 \
    bitnami/kafka:latest

Step 3: Launch your Kafka client instance

Finally we create a new container instance to launch the Kafka client and connect to the server created in the previous step:

$ docker run -it --rm \
    --network app-tier \
    -e KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181 \
    bitnami/kafka:latest kafka-topics.sh --list  --zookeeper zookeeper-server:2181

Using Docker Compose

When not specified, Docker Compose automatically sets up a new network and attaches all deployed services to that network. However, we will explicitly define a new bridge network named app-tier. In this example we assume that you want to connect to the Kafka server from your own custom application image which is identified in the following snippet by the service name myapp.

version: '2'

networks:
  app-tier:
    driver: bridge

services:
  zookeeper:
    image: 'bitnami/zookeeper:latest'
    networks:
      - app-tier
  kafka:
    image: 'bitnami/kafka:latest'
    networks:
      - app-tier
  myapp:
    image: 'YOUR_APPLICATION_IMAGE'
    networks:
      - app-tier

IMPORTANT:

Please update the YOUR_APPLICATION_IMAGE placeholder in the above snippet with your application image

In your application container, use the hostname kafka to connect to the Kafka server

Launch the containers using:

$ docker-compose up -d

Configuration

The configuration can easily be setup with the Bitnami Kafka Docker image using the following environment variables:

ALLOW_PLAINTEXT_LISTENER: Allow to use the PLAINTEXT listener. Default: no
KAFKA_PORT_NUMBER: Kafka port. Default: 9092
KAFKA_BROKER_ID: ID of the Kafka node. Default: -1
KAFKA_NUM_NETWORK_THREADS: The number of threads handling network requests.
KAFKA_NUM_IO_THREADS: The number of threads doing disk I/O. Default: 3
KAFKA_SOCKET_SEND_BUFFER_BYTES: The send buffer (SO_SNDBUF) used by the socket server. Default: 102400
KAFKA_SOCKET_RECEIVE_BUFFER_BYTES: The receive buffer (SO_RCVBUF) used by the socket server. Default: 102400
KAFKA_SOCKET_REQUEST_MAX_BYTES: The maximum size of a request that the socket server will accept (protection against OOM). Default: 104857600
KAFKA_LOGS_DIRS: A comma separated list of directories under which to store log files. Default: /opt/bitnami/kafka/data
KAFKA_DELETE_TOPIC_ENABLE: Switch to enable topic deletion or not, default value is false. Default: false
KAFKA_LISTENERS: The address the socket server listens on. Default: PLAINTEXT://:9092
KAFKA_ADVERTISED_LISTENERS: Hostname and port the broker will advertise to producers and consumers. Default: PLAINTEXT://:9092
KAFKA_LISTENER_SECURITY_PROTOCOL_MAP: Maps listener names to security protocols. Default: PLAINTEXT:PLAINTEXT,SSL:SSL,SASL_PLAINTEXT:SASL_PLAINTEXT,SASL_SSL:SASL_SSL
KAFKA_INTER_BROKER_LISTENER_NAME: Name of listener used for communication between brokers. No defaults.
KAFKA_DEFAULT_REPLICATION_FACTOR: Default replication factors for automatically created topics. Default: 1
KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR: The replication factor for the offsets topic. Default: 1
KAFKA_TRANSACTION_STATE_LOG_REPLICATION_FACTOR: The replication factor for the transaction topic. Default: 1
KAFKA_TRANSACTION_STATE_LOG_MIN_ISR: Overridden min.insync.replicas config for the transaction topic. Default: 1
KAFKA_NUM_PARTITIONS: The default number of log partitions per topic. Default: 1
KAFKA_NUM_RECOVERY_THREADS_PER_DATA_DIR The number of threads per data directory to be used for log recovery at startup and flushing at shutdown. Default: 1
KAFKA_MAX_MESSAGE_BYTES: The largest record batch size allowed by Kafka. Default: 1000012
KAFKA_LOG_FLUSH_INTERVAL_MESSAGES: The number of messages to accept before forcing a flush of data to disk. Default: 10000
KAFKA_LOG_FLUSH_INTERVAL_MS: The maximum amount of time a message can sit in a log before we force a flush. Default: 1000
KAFKA_LOG_RETENTION_HOURS: The minimum age of a log file to be eligible for deletion due to age. Default: 168
KAFKA_LOG_RETENTION_BYTES: A size-based retention policy for logs. Default: 1073741824
KAFKA_SEGMENT_BYTES: The maximum size of a log segment file. When this size is reached a new log segment will be created. Default: 1073741824
KAFKA_LOG_RETENTION_CHECK_INTERVALS_MS: The interval at which log segments are checked to see if they can be deleted. Default: 300000
KAFKA_LOG_MESSAGE_FORMAT_VERSION: Specify the message format version the broker will use to append messages to the logs. No defaults.
KAFKA_ZOOKEEPER_CONNECT: Comma separated host:port pairs, each corresponding to a Zookeeper Server. Default: localhost:2181
KAFKA_ZOOKEEPER_CONNECT_TIMEOUT_MS: Timeout in ms for connecting to zookeeper. Default: 6000
KAFKA_INTER_BROKER_USER: Kafka inter broker communication user. Default: admin. Default: admin
KAFKA_INTER_BROKER_PASSWORD: Kafka inter broker communication password. Default: bitnami
KAFKA_BROKER_USER: Kafka client user. Default: user
KAFKA_BROKER_PASSWORD: Kafka client user password. Default: bitnami
KAFKA_ZOOKEEPER_USER: Kafka Zookeeper user. No defaults
KAFKA_ZOOKEEPER_PASSWORD: Kafka Zookeeper user password. No defaults
KAFKA_CERTIFICATE_PASSWORD: Password for certificates. No defaults.
KAFKA_HEAP_OPTS: Kafka's Java Heap size. Default: -Xmx1024m -Xms1024m
KAFKA_SSL_ENDPOINT_IDENTIFICATION_ALGORITHM: The endpoint identification algorithm to validate server hostname using server certificate. Default: https

docker run --name kafka -e KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181 bitnami/kafka:latest

or using Docker Compose:

version: '2'

services:
  zookeeper:
    image: 'bitnami/zookeeper:latest'
    ports:
      - '2181:2181'
    volumes:
      - 'zookeeper_data:/bitnami/zookeeper'
  kafka:
    image: 'bitnami/kafka:0'
    ports:
      - '9092:9092'
    volumes:
      - 'kafka_data:/bitnami/kafka'
    environment:
      - KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181

volumes:
  zookeeper_data:
    driver: local
  kafka_data:
    driver: local

Security

The Bitnami Kafka docker image disables the PLAINTEXT listener for security reasons. You can enable the PLAINTEXT listener by adding the next environment variable, but remember that this configuration is not recommended for production.

ALLOW_PLAINTEXT_LISTENER=yes

In order to configure SASL authentication over SSL, you should define the proper listener by passing the following env vars:

KAFKA_LISTENERS=SASL_SSL://:9092
KAFKA_ADVERTISED_LISTENERS=SASL_SSL://:9092

You must also use your own certificates for SSL. You can drop your Java Key Stores files into /opt/bitnami/kafka/conf/certs. If the JKS is password protected (recommended), you will need to provide it to get access to the keystores:

KAFKA_CERTIFICATE_PASSWORD=myCertificatePassword

The following script can help you with the creation of the JKS and certificates. Use the same password for all them:

https://raw.githubusercontent.com/confluentinc/confluent-platform-security-tools/master/kafka-generate-ssl.sh

The following docker-compose file is an example showing how to mount your JKS certificates protected by the password certificatePassword123. Additionally it is specifying the credentials for the broker, inter-broker and zookeeper users.

version: '2'

services:
  zookeeper:
    image: 'bitnami/zookeeper:latest'
    ports:
     - '2181:2181'
    environment:
      - ZOO_ENABLE_AUTH=yes
      - ZOO_SERVER_USERS=kafka
      - ZOO_SERVER_PASSWORDS=kafka_password
  kafka:
    image: 'bitnami/kafka:latest'
    ports:
      - '9092'
    environment:
      - KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181
      - KAFKA_LISTENERS=SASL_SSL://:9092
      - KAFKA_ADVERTISED_LISTENERS=SASL_SSL://:9092
      - KAFKA_ZOOKEEPER_USER=kafka
      - KAFKA_ZOOKEEPER_PASSWORD=kafka_password
      - KAFKA_INTER_BROKER_USER=interuser
      - KAFKA_INTER_BROKER_PASSWORD=interpassword
      - KAFKA_BROKER_USER=user
      - KAFKA_BROKER_PASSWORD=password
      - KAFKA_CERTIFICATE_PASSWORD=certificatePassword123
    volumes:
      - './kafka.keystore.jks:/opt/bitnami/kafka/conf/certs/kafka.keystore.jks:ro'
      - './kafka.truststore.jks:/opt/bitnami/kafka/conf/certs/kafka.truststore.jks:ro'

InterBroker communications

By default, communications that happens between brokers are authenticated. You can provide your own credentials using this environment variables:

KAFKA_INTER_BROKER_USER: Kafka inter broker communication user. Default: admin
KAFKA_INTER_BROKER_PASSWORD: Kafka inter broker communication password. Default: bitnami

Kafka client configuration

By default, any Kafka client needs to authenticate before can connect to a broker. You can provide your own credentials using this environment variables:

KAFKA_BROKER_USER: Kafka client user. Default: user
KAFKA_BROKER_PASSWORD: Kafka client user password. Default: bitnami

Kafka ZooKeeper client configuration

In order to authenticate Kafka against a Zookeeper server with SASL authentication you should provide the next environment variables:

KAFKA_ZOOKEEPER_USER: Kafka Zookeeper user. No defaults.
KAFKA_ZOOKEEPER_PASSWORD: Kafka Zookeeper user password. No defaults.

Below you can see a complete Docker Compose example:

version: '2'

services:
  zookeeper:
    image: 'bitnami/zookeeper:latest'
    ports:
     - '2181:2181'
    environment:
      - ZOO_ENABLE_AUTH=yes
      - ZOO_SERVER_USERS=kafka
      - ZOO_SERVER_PASSWORDS=kafka_password
  kafka:
    image: 'bitnami/kafka:latest'
    ports:
      - '9092'
    environment:
      - KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181
      - KAFKA_LISTENERS=SASL_SSL://:9092
      - KAFKA_ADVERTISED_LISTENERS=SASL_SSL://:9092
      - KAFKA_ZOOKEEPER_USER=kafka
      - KAFKA_ZOOKEEPER_PASSWORD=kafka_password

Connecting services with security enabled

In order to get the required credentials to consume and produce messages you need to provide the credentials in the client. If your Kafka client allows it, use the credentials you've provided.

While producing and consuming messages using the bitnami/kafka image, you'll need to point to the consumer.properties and/or producer.properties file, which contains the needed configuration to work. You can find this files in the /opt/bitnami/kafka/conf directory

Use this to generate messages using a secure setup

export KAFKA_OPTS="-Djava.security.auth.login.config=/opt/bitnami/kafka/conf/kafka_jaas.conf"
kafka-console-producer.sh --broker-list 127.0.0.1:9092 --topic test --producer.config /opt/bitnami/kafka/conf/producer.properties

Use this to consume messages using a secure setup

export KAFKA_OPTS="-Djava.security.auth.login.config=/opt/bitnami/kafka/conf/kafka_jaas.conf"
kafka-console-consumer.sh --bootstrap-server 127.0.0.1:9092 --topic test --consumer.config /opt/bitnami/kafka/conf/consumer.properties

If you use other tools to use your Kafka cluster, you'll need to provide the required information. You can find the required information in the files located at /opt/bitnami/kafka/conf directory.

Setting up a Kafka Cluster

A Kafka cluster can easily be setup with the Bitnami Kafka Docker image using the following environment variables:

KAFKA_ZOOKEEPER_CONNECT: Comma separated host:port pairs, each corresponding to a Zookeeper Server.

Create a Docker network to enable visibility to each other via the docker container name

docker network create app-tier --driver bridge

Step 1: Create the first node for Zookeeper

The first step is to create one Zookeeper instance.

docker run --name zookeeper \
  --network app-tier \
  -p 2181:2181 \
  bitnami/zookeeper:latest

Step 2: Create the first node for Kafka

The first step is to create one Kafka instance.

docker run --name kafka1 \
  --network app-tier \
  -e KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181 \
  -p 9092:9092 \
  bitnami/kafka:latest

Step 2: Create the second node

Next we start a new Kafka container.

docker run --name kafka2 \
  --network app-tier \
  -e KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181 \
  -p 9092:9092 \
  bitnami/kafka:latest

Step 3: Create the third node

Next we start another new Kafka container.

docker run --name kafka3 \
  --network app-tier \
  -e KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181 \
  -p 9092:9092 \
  bitnami/kafka:latest

You now have a Kafka cluster up and running. You can scale the cluster by adding/removing slaves without incurring any downtime.

With Docker Compose, topic replication can be setup using:

version: '2'

services:
  zookeeper:
    image: 'bitnami/zookeeper:latest'
    ports:
     - '2181:2181'
  kafka1:
    image: 'bitnami/kafka:latest'
    ports:
      - '9092'
    environment:
      - KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181
  kafka2:
    image: 'bitnami/kafka:latest'
    ports:
      - '9092'
    environment:
      - KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181
  kafka3:
    image: 'bitnami/kafka:latest'
    ports:
      - '9092'
    environment:
      - KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181

Then, you can create a replicated topic with:

root@kafka1:/# /opt/bitnami/kafka/bin/kafka-topics.sh --create --zookeeper zookeeper:2181 --topic mytopic --partitions 3 --replication-factor 3
Created topic "mytopic".

root@kafka1:/# /opt/bitnami/kafka/bin/kafka-topics.sh --describe --zookeeper zookeeper:2181 --topic mytopic
Topic:mytopic   PartitionCount:3        ReplicationFactor:3     Configs:
        Topic: mytopic  Partition: 0    Leader: 2       Replicas: 2,3,1 Isr: 2,3,1
        Topic: mytopic  Partition: 1    Leader: 3       Replicas: 3,1,2 Isr: 3,1,2
        Topic: mytopic  Partition: 2    Leader: 1       Replicas: 1,2,3 Isr: 1,2,3

Configuration

The image looks for configuration in the conf/ directory of /opt/bitnami/kafka.

docker run --name kafka -v /path/to/server.properties:/opt/bitnami/kafka/conf/server.properties bitnami/kafka:latest

After that, your changes will be taken into account in the server's behaviour.

Step 1: Run the Kafka image

Run the Kafka image, mounting a directory from your host.

Using Docker Compose:

version: '2'

services:
  kafka:
    image: 'bitnami/kafka:latest'
    ports:
      - '9092:9092'
    volumes:
      - /path/to/server.properties:/opt/bitnami/kafka/conf/server.properties

Step 2: Edit the configuration

Edit the configuration on your host using your favorite editor.

vi /path/to/server.properties

Step 3: Restart Kafka

After changing the configuration, restart your Kafka container for changes to take effect.

docker restart kafka

or using Docker Compose:

docker-compose restart kafka

Logging

The Bitnami Kafka Docker image sends the container logs to the stdout. To view the logs:

docker logs kafka

or using Docker Compose:

docker-compose logs kafka

You can configure the containers logging driver using the --log-driver option if you wish to consume the container logs differently. In the default configuration docker uses the json-file driver.

Maintenance

Backing up your container

To backup your data, configuration and logs, follow these simple steps:

Step 1: Stop the currently running container

docker stop kafka

or using Docker Compose:

docker-compose stop kafka

Step 2: Run the backup command

We need to mount two volumes in a container we will use to create the backup: a directory on your host to store the backup in, and the volumes from the container we just stopped so we can access the data.

docker run --rm -v /path/to/kafka-backups:/backups --volumes-from kafka busybox \
  cp -a /bitnami/kafka:latest /backups/latest

or using Docker Compose:

docker run --rm -v /path/to/kafka-backups:/backups --volumes-from `docker-compose ps -q kafka` busybox \
  cp -a /bitnami/kafka:latest /backups/latest

Restoring a backup

Restoring a backup is as simple as mounting the backup as volumes in the container.

docker run -v /path/to/kafka-backups/latest:/bitnami/kafka bitnami/kafka:latest

or using Docker Compose:

version: '2'

services:
  kafka:
    image: 'bitnami/kafka:latest'
    ports:
      - '9092:9092'
    volumes:
      - /path/to/kafka-backups/latest:/bitnami/kafka

Upgrade this image

Bitnami provides up-to-date versions of Kafka, including security patches, soon after they are made upstream. We recommend that you follow these steps to upgrade your container.

Step 1: Get the updated image

docker pull bitnami/kafka:latest

or if you're using Docker Compose, update the value of the image property to bitnami/kafka:latest.

Step 2: Stop and backup the currently running container

Before continuing, you should backup your container's data, configuration and logs.

Follow the steps on creating a backup.

Step 3: Remove the currently running container

docker rm -v kafka

or using Docker Compose:

docker-compose rm -v kafka

Step 4: Run the new image

Re-create your container from the new image, restoring your backup if necessary.

docker run --name kafka bitnami/kafka:latest

or using Docker Compose:

docker-compose up kafka

Notable Changes

1.1.0-r41

Configuration is not persisted anymore. It should be mounted as a volume or it will be regenerated each time the container is created.
Dummy certificates are not used anymore when the SASL_SSL listener is configured. These certificates must be mounted as volumes.

0.10.2.1-r3

The kafka container has been migrated to a non-root container approach. Previously the container run as root user and the kafka daemon was started as kafka user. From now own, both the container and the kafka daemon run as user 1001. As a consequence, the configuration files are writable by the user running the kafka process.

0.10.2.1-r0

New Bitnami release

Contributing

We'd love for you to contribute to this container. You can request new features by creating an issue, or submit a pull request with your contribution.

Issues

If you encountered a problem running this container, you can file an issue. For us to provide better support, be sure to include the following information in your issue:

Host OS and version
Docker version (docker version)
Output of docker info
Version of this container (echo $BITNAMI_IMAGE_VERSION inside the container)
The command you used to run the container, and any relevant output you saw (masking any sensitive information)

License

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

flanciskinho/bitnami-docker-kafka

What is Kafka?

TL;DR;

Docker Compose

Why use Bitnami Images?

How to deploy Apache Kafka in Kubernetes?

Why use a non-root container?

Supported tags and respective Dockerfile links

Get this image

Persisting your data

Connecting to other containers

Using the Command Line

Step 1: Create a network

Step 2: Launch the Zookeeper server instance

Step 2: Launch the Kafka server instance

Step 3: Launch your Kafka client instance

Using Docker Compose

Configuration

Security

InterBroker communications

Kafka client configuration

Kafka ZooKeeper client configuration

Connecting services with security enabled

Setting up a Kafka Cluster

Step 1: Create the first node for Zookeeper

Step 2: Create the first node for Kafka

Step 2: Create the second node

Step 3: Create the third node

Configuration

Step 1: Run the Kafka image

Step 2: Edit the configuration

Step 3: Restart Kafka

Logging

Maintenance

Backing up your container

Step 1: Stop the currently running container

Step 2: Run the backup command

Restoring a backup

Upgrade this image

Step 1: Get the updated image

Step 2: Stop and backup the currently running container

Step 3: Remove the currently running container

Step 4: Run the new image

Notable Changes

1.1.0-r41

0.10.2.1-r3

0.10.2.1-r0

Contributing

Issues

License

Supported tags and respective `Dockerfile` links